Our goal is to investigate the intrathymic genesis of autoimmunity. Specific investigations are based on a new concept of the role of the thymus in at least one form of autoimmunity, the neuromuscular disease myasthenia gravis (MG). Most thymic epithelial cells express Ia constitutively, and medullary epithelial cells express nicotinic acetylcholine receptors (AChR), the antigen of MG. Much is known about the role of anti-AChR autoantibodies in causing failure of neuromuscular transmission in MG, but nothing is known about why these antibodies arise. In MG the thymus is a major site of anti-AChR antibody production. Its removal frequently is followed by remission of the disease. The thymus undergoes neoplastic transformation in 15% of MG patients. Studies outlined in this proposal concern the role of thymic epithelial cells in initiating autoimmunity to AChR. Hypotheses to be tested are that: i) AChR associated with thymic epithelial cells is the primary immunogen of spontaneously acquired MG; ii) this immunogenicity is promoted by activation of vertically transmitted proviral genes; iii) production of anti-AChR autoantibodies reflects an anti-tumor/virus immune response that is progressing in situ in the thymus; iv) some epitopes of the AChR expressed on thymic epithelial cells are neuroectoderm-specific and are expressed selectively on cranial (oculobulbar) muscles. The long-term goal is to find a basis for new and more effective methods of treating MG. Materials to be tested include normal thymus tissues from mice, rats and humans, and cryopreserved thymus tissue and thymomas from MG patients. The AKR/J strain of mouse, which has a 90% incidence of thymic tumors and a paradoxical susceptibility to experimental autoimmune MG (EAMG), will be investigated as a novel model of the relationship between thymoma and MG. Methods include culture of thymic epithelial cells and thymomas, probing immunologically and with 32P-cDNA for coexpression of AChR and viral or onc gene products, generation of T cell clones, testing ability of thymic epithelial cells to present AChR, testing EAMG susceptibility in AKR/J mice with thymoma suppressed by neonatal treatment with anti-gp70 antibodies, serologic comparisons of the antigenicity of AChR in thymus and cranial and limb muscles.